The present invention relates to a normally closed liquid crystal display device, particularly to such a liquid crystal display device incorporating an in-plane switching (IPS) mode which is operated by applying an electric field to the liquid crystal layer in the direction substantially parallel to the substrate plane.
In liquid crystal display devices, the dark level of dark display is a decisive factor for the contrast ratio. The principal causes of worsening the dark level of dark display are the improper conditions of pixel and electrode ends, low frame response and light leakage at the periphery of the spacer.
Variations in thickness of the liquid crystal layer in these liquid crystal display devices lead to serious deterioration of display qualities such as reduction of contrast, nonuniformity of luminance and color shading, so that the spacers are required for keeping the uniform thickness of the liquid crystal layer.
Provision of such spacers, however, is causative of light leakage as it disturbs alignment of the liquid crystal molecules around the spacer surface. The higher the in-plane distribution density of spacers, the more effective for suppressing variation of thickness of the liquid crystal layer, but at the same time this may invite a greater decrease of contrast ratio. Also, light leakage at the periphery of the spacer tends to provoke a sense of glaringness which hinders appearance of uniform blackness of the picture when the display mode changes from dark to gray scale, and may also cause nonuniformity of luminance due to localized distribution of the spacers.
In the normally closed liquid crystal display devices of the twisted nematic (TN) mode in which an electric field is applied along the direction perpendicular to the substrates, a method for minimizing light leakage at the spacer periphery by incorporating a mechanism for vertically aligning the liquid crystal molecules in the spacer surface has been proposed (JP-A-4-177324, etc.). This method is indeed effective in the normally closed liquid crystal display devices of the TN mode, but no disclosure is made on the technique for reducing or eliminating light leakage which occurs specifically in the IPS mode normally closed liquid crystal display devices to which the conception of the present invention is directed.
An object of the present invention is to provide an IPS mode normally closed liquid crystal display device of high contrast ratio, which is freed of the problem of light leakage caused by the spacer beads in this type of liquid crystal display devices.
Accordingly, the present invention provides an IPS mode normally closed liquid crystal display device comprising:
a pair of substrates at least one of which is transparent,
a liquid crystal layer disposed between said pair of substrates,
a group of electrodes for applying to said liquid crystal layer an electric field substantially parallel to the substrate plane and one or more active elements connected to said electrodes, said group of electrodes and active elements being formed on one of said pair of substrates,
an alignment control film formed on said pair of substrates, and
an optical means for changing the optical properties according to the state of molecular alignment of said liquid crystal layer,
wherein the thickness of said liquid crystal layer is controlled to be substantially uniform by the spacer beads dispersed and held between said pair of substrates, a pretilt angle between said alignment control film and liquid crystals is 4xc2x0 or less, and a contact angle between said liquid crystals and said spacer is 0xc2x0 to 60xc2x0.
The present invention also provides a liquid crystal display device of the type recited, which is freed of the problem of light leakage due to the presence of the spacer beads by disposing between said spacer and said liquid crystal layer a film provided with a liquid crystal alignment controlling function by irradiation with polarized light.